1. Field of the Invention
This invention relates to a separately excited DC brush motor wherein a field and an armature current can be controlled separately, and more particularly to a field current control method when the overrotation or reverse rotation of motors is detected.
2. Description of the Related Art
FIG. 7 shows a separately excited DC brush motor and its control circuits. An armature current control circuit for feeding power to the armatures 61 of motors ML and MR, and a field current control circuit for feeding power to a field system 62 form separate systems and are independently controllable except that both the control circuits share a battery B2 between them. The armature current control circuit includes a capacitor 63, a diode 64, one FET 65, an armature current sensor 66a, and a battery voltage sensor 66b, the armature current being turned on electricity in only one direction. The field current control circuit includes a capacitor 67, four FETs 68 . . . , and a field current sensor 69. And the field current control circuit is capable of varying the direction of the field current (the direction of magnetic flux of the field system) and the intensity of the field current (intensity of the magnetic flux of the field system) by controlling the four FETs 68 . . . under PWM control.
When front wheels of a front-and-rear wheel drive vehicle are driven by an engine with the rear wheels driven by separately excited brush motors ML and MR, the motors ML and MR may be overrotated or reversely rotated by the driving force reversely transmitted from the rear wheels. When the motors ML and MR are overrotated, the motors ML and MR function as generators for generating a voltage higher than the voltage of the battery and this may result in damaging the current control elements of armature current control circuits connected to the armatures 61 of the motors ML and MR. In a case where the dielectric strength of the armature current control circuits is increased in order to prevent such damage, there arises a problem of increasing costs. Even when the motors ML and MR are reversely rotated, the motors ML and MR that function as generators allow an excessive current to flow into the diode 64 even when the FET 65 of the armature current control circuit is turned off. In a case where the current capacitance and heat radiability of the diode 64 are increased, there also develops a problem of increasing costs.
An object of the present invention made in view of the situation above is to prevent an armature current control circuit from being damaged when motors are overrotated or reversely rotated by the driving force transmitted from a driven portion.
In order to accomplish the object, according to the invention in a first aspect of this invention, proposed is a field current control method in motors such as a separately excited DC brush motor wherein a field current and an armature current are separately controllable. Also the method comprises the step of reducing the field current when overrotation or reverse rotation of motors is detected.
With the arrangement above, since the field current is reduced when the overrotation and reverse rotation of the motors is detected, the motors are prevented from functioning as generators for generating an excessive current and an excessive voltage, whereby the armature current control circuit of the motors can be prevented from being damaged without particularly increasing the dielectric strength and current capacitance.
According to the invention in a second aspect of this invention, proposed is, in addition to the arrangement in the first aspect, a field current control method has feature that the overrotation is detected from a rise in the voltage of a battery for feeding power to motors.
With the arrangement above, since the overrotation of the motors is detected from a rise in the voltage of the battery for feeding power to the motors, the overrotation can be detected exactly without providing a sensor for detecting the number of rotations of the motors.
According to the invention in a third aspect of this invention, proposed is, in addition to the arrangement in the first aspect, a field current control method by which the overrotation is detected from backward flow of the armature current of the motors.
With the arrangement above, since the overrotation of the motors is detected from the backward flow of the armature current of the motors, the overrotation can be detected exactly without providing a sensor for detecting the number of rotations of the motors.
According to the invention in a fourth aspect of this invention, proposed is, in addition to the arrangement in the first aspect, a field current control method by which the reverse rotation is detected according to the actually measured value of the armature current or the PWM value of the armature current of the motors.
With the arrangement above, since the reverse rotation of the motors is detected according to the actually measured value of the armature current or the PWM value of the armature current of the motors, the overrotation can be detected exactly without providing a sensor for detecting the number of rotations of the motors.